It is conventional in the papermaking industry to use suction pipe systems and in particular suction pipes with elongated slots in alignment with a felt. Each suction pipe is positioned so that the felt passes over the slot and the suction causes dewatering of the felt. The water collects within the suction pipe and is directed to an appropriate collection location. Suitable separators can be employed to facilitate collection of the water drawn from the felt by the vacuum dewatering system.
There are several basic types of vacuum pumps presently used in dewatering systems with the choice being dependent on a variety of parameters including cost, machine deficiency and the type of papermaking machinery being utilized. Three basic types of vacuum pumps used in the paper industry are the liquid ring pump, the positive displacement pump, and the centrifugal exhauster or blower. Each type has its advantages and disadvantages with respect to one another and different maximum efficiency values on air flow versus vacuum settings. Therefore, it is important to select not only a particular type of vacuum pump for a given application, but also with size, port openings, number of stages, and other criteria, for the lowest horsepower for unit air flow requirement. Lower horsepower naturally reduces manufacturing, assembly and use costs as well as producing lower energy consumption which is of extreme concern today.
Other factors that always have to be considered in the selection of a vacuum pump system besides the lower horsepower requirements are purchase price, total installation cost, maintenance, seal water requirements, amount of liquid with incoming air flow, and presence of contamination such as solids or fibers. In other words, one type of vacuum pump may look good from a horsepower standpoint, but because of the above other considerations, may not be practical or the total system cost may be more expensive than using another type pump.
In considering the above parameters, an important balancing criteria is based upon sufficient power to permit the use of a felt for dewatering purposes over the longest possible time before replacement is required. It is well known that the felt will wear over a period of time in use and will ultimately have to be replaced. However, the felt also undergoes a reduction in permeability as it is used over a period of time for dewatering purposes. This reduction in permeability naturally affects the efficiency of the dewatering system. Consequently, vacuum pumps of substantial horsepower are utilized in present dewatering systems so that the felts can be used for a longer period of time even after the permeability has been substantially reduced. Naturally the larger horsepower vacuum pump is considerably over sized for the system when the felt is new causing the system to be inefficient and more costly than necessary during a substantial portion of the time a felt is employed. It is only when the permeability has been reduced sufficiently for the additional horsepower to be needed that it is utilized.
Alternatively, felts can be more frequently replaced but this is a costly and time consuming procedure which is undesirable in the industry.
It should also be noted that even with the oversized vacuum pump in regard to horsepower, the additional horsepower is often not sufficient to effectively dewater with the use of a single suction pump and a fixed slot width. It has been shown that increased dwell time is also an effective means of efficiently dewatering as well as increasing the pressure differential.